This invention relates generally to an improved way to pour molten metal used in a casting operation, and more particularly to minimize the metal damage due to filling of shot sleeve of a horizontal high pressure die casting machine by using bottom filling of the shot sleeve combined with sequential rotation of a pouring ladle and the shot sleeve.
Low process cost, close dimensional tolerances (near-net-shape) and smooth surface finishes are all desirable attributes that make high pressure die casting (HPDC) a widely used process for the mass production of metal components. By way of example, manufacturers in the automobile industry use HPDC to produce near-net-shape aluminum alloy castings for engine and transmission components. In a typical HPDC process, molten metal is introduced into shaped mold cavities through two metal transfer steps: a (first) low pressure tilt pour from a ladle to a filler tube (called a shot sleeve), and a (second) high pressure injection (such as upon movement of a piston in the tube) into the gating/casting cavity.
Aluminum alloy castings are sensitive to molten metal delivery speed. When the delivery speed is too low, misruns and cold shuts may result; when it is too high, turbulent flow can entrap air or other gases that can in turn lead to oxide formations, as well as form surface molten aluminum that oxidizes when it comes in contact with ambient air. Both forms of oxides are commonly referred to as dross. The concern over higher speed HPDC operations—while more efficient for large-scale production than their low-speed counterparts—is particularly acute considering that the high velocities are an inherent part of the higher delivery pressures. Both the entrapped (i.e., bi-film) and surface (i.e., top-layer) dross mix and subsequently solidify with the rest of the molten metal, which in turn leads to inclusions and highly porous regions that adversely impact structural and mechanical properties of the cast component.
Research has shown that the entrained air (i.e., bi-film) variant of dross can arise if the velocity of the liquid metal is sufficiently high, and that such a velocity is believed to be between 0.45 m/s and 0.5 m/s for Al, Mg, Ti and Fe alloys. See, for example, Campbell, Castings (Elsevier Butterworth-Heinemann, 2003). Thus, it is desirable to keep metal delivery speeds under this critical velocity to significantly reduce the number of oxides being formed in the casting. U.S. Pat. No. 8,522,857—which is owned by the Assignee of the present invention and incorporated herein in its entirety by reference—evidences additional research that correlates the delivery location of the molten metal from the ladle to significant reductions in turbulence and other dross-inducing events. That approach employed a side-pour ladle configuration that takes advantage of the fact that metal at the bottom of the ladle is substantially free from dross and other foreign material, as well as eliminates the exposed plunging metal stream during pour basin filling. Such a ladle design has been shown minimize turbulence in ways not possible with traditional tilt-pour molding processes. Nevertheless, additional innovations are needed to take full advantage of a side pour ladle used in the filling of an HPDC shot sleeve.